Such a resonator of small dimensions has been disclosed in the prior art document U.S. Pat. No. 6,700,313 filed in the name of the name Assignee, which is enclosed herewith by way of reference. Resonator 10 represented on FIG. 13, which is intended to be mounted in a case, includes a tuning fork shaped part with two parallel arms 12, 14 connected to each other by a linking part 16 and carrying electrodes 20, 22 to make them vibrate, these electrodes being connected to connecting pads 28, 30 intended to be electrically connected to the exterior of the case. Resonator 10 also includes a central arm 18 attached to linking part 16 and located between arms 12, 14 of the tuning fork shaped part, substantially equidistant from them, this central arm 18 having a greater mass than that of the arms of the tuning fork shaped part and connected pads 28, 30 are carried by this central arm. This resonator is mounted in a case of parallelepiped shape by fixing its central arm 18 to at least one support secured to the bottom of the case.
The above described kind of resonators, called hereinafter three arm resonators, allows breaking away from many problems encountered with conventional tuning fork resonators.
One of these problems is that the dimensional and functional features of the conventional tuning fork resonators are optimised for mounting them in metallic cases and not in ceramic cases. For example, the ratio between their length and their width is ill suited to the manufacture of such cases, in particular when the cases are of the SMD (Surface Mounting Device) type, i.e. meant to be automatically mounted on hole-free printed circuit boards. Because of this, a conventional tuning fork resonator and its connection to the case do not have very good resistance to shocks.
Further conventional tuning fork resonators are liable to tip towards the bottom of the case when it is fixed onto a step of the case.
Furthermore, since the thermal expansion coefficients of ceramic material and quartz are different enough to create mechanical stresses in the resonator which can not only be felt in the arms of the tuning fork and disturb the working of the resonator when the temperature changes but can also break the solder or detach connection pads of the resonator from those of the case and alter or even cut the electric connection between the resonator electrodes and the external contact pads of the case.
For the same reason, if there is an incipient fracture on the side of base of the tuning fork where connection pads are situated, a significant temperature variation can even break the tuning fork.
Finally, when the resonator is vacuum packed, the vacuum is never perfect and the stirring of the atmosphere by the tuning fork arms when the resonator vibrates can modify the operating parameters of the resonator when its arms come close to each other, more so than in the case of a metallic case.
Thus, three arm resonators enable satisfactory solutions to be brought to the various problems posed by conventional tuning fork resonators. Nevertheless although these three arm resonators have proved to be advantageous, miniaturization requirements as well as energy consumption required to maintain vibrations of the tuning fork are always critical issues for mobile phone or wristwatch applications.